Patent Literature 1 mentioned below discloses an X-ray generator for generating ion gas by irradiating air with X-rays. An X-ray tube used in this X-ray generator includes a main body which is a cylindrical package (or bulb). On the inside of the package, electrons emitted from a filament are focused by a focus and are collided with an X-ray target, thereby generating X-rays. The X-rays then pass through an output window (i.e. an X-ray transmission window) and exit from the package to the outside.
FIG. 4 shows a cross-sectional view of an X-ray tube which is similar to the X-ray tube of Patent Literature 1 mentioned above and which is a so-called circular-tube-type X-ray tube (hereinafter called “circular X-ray tube”) having a main body which is a cylindrical package 100 made of glass. The cylindrical package 100 has a circular opening portion at its one end face. This opening portion is closed by an X-ray transmission window 101 made of a beryllium film so that the inside of the cylindrical package 100 is maintained in a high-vacuum state. On the inside of the cylindrical package 100, an X-ray target 102 is provided on an inner surface of the X-ray transmission window 101. Also, a cathode 103 as an electron source and a control electrode 104 are provided on the side of the other end face of the cylindrical package 100. The electrons emitted from the cathode 103 are accelerated by the control electrode 104, focused and collided with the X-ray target 102, thereby radiating the X-rays from the X-ray transmission window 101 to the outside of the cylindrical package 100. In FIG. 4, the X-rays radiated through the X-ray transmission window 101 to the outside of the cylindrical package 100 are indicated by a reference sign X, and a center of the emission of the X-rays at the X-ray transmission window 101 is indicated by a reference sign P.